| 1. |
- Dyachenko, O G, et al.
(författare)
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Structure and properties of Ba6 − xLnxNb10O30, Ln = La, Ce and Nd compounds
- 1997
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Ingår i: Materials research bulletin. - 0025-5408 .- 1873-4227. ; 32:4, s. 409-419
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Tidskriftsartikel (refereegranskat)abstract
- Ba6 − xLnxNb10O30 compounds, Ln = La, Ce and Nd, with a tetragonal tungsten bronze-type structure have been synthesized and were characterized by X-ray powder diffraction and EDS analysis. The homogeneity regions of the obtained compounds were found to be 0 ≤ x ≤ 2 for La, and 0 ≤ x ≤ 1.5 for Ce and Nd. The crystal structures of Ba5LaNb10O30 and Ba4La2Nb10O30 were refined using X-ray powder diffraction data. Refinement indicated the presence of vacancies in the positions. This observation was supported by oxidation studies of the La-substituted compounds. The temperature dependence of the normalized resistivity of the compounds showed a nonmetallic behavior. The increase in the ratio with decreasing temperature is less distinct for the phases with the higher concentration of rare-earth cation.
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| 2. |
- Felser, C, et al.
(författare)
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Metal valence states in Eu0.7NbO3, EuNbO3, and Eu2Nb5O9 by TB-LMTO-ASA band-structure calculations and resonant photoemission spectroscopy
- 1998
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 57, s. 1510-1514
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structures of Eu2Nb5O9, EuNbO3, and Eu0.7NbO3 have been investigated by photoemission and total-yield spectroscopy with synchrotron radiation, and in the case of Eu2Nb5O9 by tight-binding linear muffin-tin orbital (LMTO) band-structure calculations. A central question for reduced europium niobates is that of the valence of Eu and Nb. Both europium and niobium atoms can appear in different valence states so that various electronic configurations in the title compounds are possible. For this reason, the valence band was studied by the resonant Eu 4d⃗4f technique to determine the Eu valence. The final-state 4f6 multiplet of divalent Eu is dominant in all spectra. Since there are no 4f density of states at the Fermi level, valence fluctuations are not expected. The niobium valence states were investigated by core-level spectroscopy. We found only one 3d5/23d3/2 doublet for the Nb 3d core level in EuNbO3 and Eu0.7NbO3, while in Eu2Nb5O9, two 3d doublets have been observed, corresponding to two chemically distinct niobium atoms in this compound. The 3d5/2 peak in EuNbO3 is assigned to the +4 nominal valence state at a binding energy of 209.7 eV. The doublet of Eu0.7NbO3 is observed at 0.5 eV higher binding energy (at 210.2 eV), which then corresponds to a nominal Nb+4+δ chemical state. In Eu2Nb5O9, the valence of Nb in the NbO6 octahedra is less than +5 and in the Nb6O12 clusters is close to +2 as expected. This is in accordance with the LMTO band-structure calculations
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| 3. |
- Grins, Jekab, et al.
(författare)
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High-resolution electron microscopy of a Sr-containing sialon polytypoid phase
- 1999
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 19, s. 2723-2730
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Tidskriftsartikel (refereegranskat)abstract
- A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi10-xAl18+xN32-xOx (x approximate to l) has been found in the Sr-Si-Al-O-N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5.335(5)approximate to root 3.a(AIN) and c= 79.1(1)Angstrom approximate to 30.c(AIN). The Sr atoms ave located in layers M-Sr-M, M=(Si/Al), at the origin of the unit cell with 12 X= (O,N) atoms around it, at distances of similar to 3 Angstrom, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M = (Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M-Sr-M layers alternate with normally eight-layer-thick AIN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AIN blocks, similar to that proposed to occur in Si-Al-O-N polytypoid phases. The model with one Sr layer and 10 M = (Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.
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| 4. |
- Grins, Jekabs, et al.
(författare)
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Synthesis and structural characterisation of MnWN2 prepared by ammonolysis of MnWO4
- 1995
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Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 5, s. 571-575
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Tidskriftsartikel (refereegranskat)abstract
- A new ternary transition-metal nitride, the composition of which is close to MnWN2, has been synthesized by ammonolysis of X-ray amorphous MnWO4 at 800 degrees C. The Guinier-Hagg powder pattern could be indexed with a hexagonal cell with a = 2.9213(3) Angstrom and c = 10.957(2) Angstrom. Although no superstructure reflections could be observed in the X-ray powder diffraction pattern, such reflections were revealed by electron diffraction. In most of the crystallites investigated, the relationship between supercell and the hexagonal (sub)cell was found to be a(super) = root 3a(hex). A Rietveld structure determination, based on space group P6(3)me (no. 186), showed that the metal atoms were approximately close-packed in an ABAC... sequence with Mn at site 2(a) and W at site 2(d), The lowest R(F) value (10%) was obtained for an ordered structure model with the Mn and N atoms occupying 2(a) and the W and N at sites 2(b). Measurements of the magnetic susceptibility, and of the electrical resistivity, indicated a metallic behaviour of MnWN2
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| 5. |
- Grins, Jekabs, et al.
(författare)
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Synthesis, Structure and Magnetic Susceptibility of the Oxynitride Spinel Mn2(MnTa3)N6-δO2+δ, 0 ≤ δ ≤ 1
- 1995
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 117:1, s. 48-54
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Tidskriftsartikel (refereegranskat)abstract
- The oxynitride spinel Mn-2(MnTa3)N6-deltaO2+delta, with 0 less than or equal to delta less than or equal to 1, has been synthesized at 1175 K by ammonolysis of a mixture of a Ta-containing xerogel and Mn(OAc)(2) . 4H(2)O. The N content was determined by combustion analysis and thermogravimetric oxidation, yielding a composition confined between Mn-2(MnTa3)N6O2 (delta = 0) and Mn-2(MnTa3)N5O3 (delta = 1). The structure is cubic, with space group Fd3m and a = 8.8353(3) Angstrom. It was refined using the Rietveld technique and neutron powder diffraction data collected at room temperature and 15 K, to R(F) = 2.9 and 3.8%, respectively. The tetrahedral sites are occupied only by Mn atoms and the octahedral sites statistically by 25% Mn and 75% Ta atoms. The N and O atoms are randomly distributed over the anion sites. The magnetic susceptibility exhibits a maximum at 29 K and a Curie-Weiss behavior at higher temperatures with theta(a) = -250(20) K and mu(eff) = 5.7(2) Bohr magnetons per Mn atom. The neutron powder diffraction data collected at 15 K showed no evidence of magnetic ordering. A NaCl-type phase with a = 4.4382(2) Angstrom and tentative composition Mn0.8Ta0.2(O,N) was observed in preparations at 1175 K. A hexagonal Mn4Ta2(O,N)(x) phase with cell dimensions a = 5.3024(4) Angstrom, c = 14.493(2) Angstrom was obtained at 973 K. (C) 1995 Academic Press, Inc.
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| 6. |
- Hannerz, H, et al.
(författare)
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Structure and magnetic susceptibility of MnNb3O6
- 1999
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Ingår i: Solid State Sciences. - 1293-2558 .- 1873-3085. ; 1, s. 567-575
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Tidskriftsartikel (refereegranskat)abstract
- The mixed valence state niobium compound MnNb3O6 was found while studying phase formations in the system MnO-Nb2O5-NbO. It is isostructural with AxNb3O6, x ≤ 1 and A = Na, Ca. Single crystals were obtained by heating MnC2O4 · 2H2O and Nb2O5 in a flow of H2 at 1300 °C. Monophasic samples were also prepared by heating stoichiometric mixtures of MnO, Nb2O5 and Nb in niobium ampoules under Ar(g) at 1100 °C. The crystal structure of MnNb3O6 (Immm, Z = 4, a = 7.1057(5), b = 10.1420(6), c = 6.5341(5) Å) was refined, using singlecrystal MoKα X-ray diffraction data, to a weighted R value of 0.018 for 329 unique reflections. The structure contains undulating layers of NbO6 octahedra of the type α2[NbO6/2]in the ac-plane, with the octahedra sharing edges along [001]and corners along [100]. Between the layers there are columns along [001]of edge-sharing square MnO8 prisms alternating with columns containing Nb2O8 clusters with an Nb-Nb distance of 2.6163(5) Å. The magnetic susceptibility shows a Curie-Weiss behaviour: χM = C/(T+θ) with θ ≈ −22 K and μeff = 6.0(1) μB for T ≥ ca. 35 K, with a small deviation from this dependence at lower temperatures, indicating Mn2+ ions with localised magnetic moments and antiferromagnetic interactions.
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| 7. |
- Hannerz, H, et al.
(författare)
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Transmission electron microscopy and neutron powder diffraction studies of GdFeO3 type SrNbO3
- 1999
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 147, s. 421-428
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Tidskriftsartikel (refereegranskat)abstract
- Stoichiometric SrNbO3; with a GdFeO3 type structure was synthesized between 1300 and 1550°C in Ar-filled and sealed niobium ampoules. Its crystal structure was refined using time-of-flight neutron diffraction data. Space group Pnma: a=√2·ap=5.6894(2) Å; b=2·ap=8.0684(1) Å; c=√2·ap=5.6944(2) Å; RF2=0.020; and RP=0.034. High-resolution electron microscopy and electron diffraction studies frequently revealed structural defects in SrNbO3. X-ray and electron diffraction studies showed cubic symmetry for SrNbO3 synthesized below 1300°C and for compounds with nominal composition Sr0.8NbO3 synthesized between 1200 and 1550°C. SrNbO3 was found to be temperature independently paramagnetic
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| 8. |
- Hörlin, Tom, et al.
(författare)
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Extended defect structures in zinc oxide doped with iron and indium
- 1998
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Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 8, s. 2465-2473
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Tidskriftsartikel (refereegranskat)abstract
- The effects of iron- and indium oxide doping on the structure: and magnetic susceptibility of ZnO have been studied. The nominal compositions were InxFe2-xO3(ZnO)n with 0 less than or equal to x less than or equal to 1 and n= 23, 48 and 98. Magnetic measurements showed the iron-doped samples to be paramagnetic, with a behaviour indicating antiferromagnetic coupling between the iron-ions. HREM studies showed that indium and iron are incorporated as layer defects of two kinds. One type forms cubic close packed (ccp) planes perpendicular to the c axis, and the other appears as-corrugated layers inserted between the former. The folds in the corrugated layers consist of alternating (114) and (11 (4) over bar) planes. Analytical transmission electron microscopy studies revealed that indium prefers the ccp layers, whereas iron can be found in both types of defects. Structural models based upon the experimental results are presented.
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| 9. |
- Istomin, S Y, et al.
(författare)
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Perovskite-Type Ca1−xSrxNbO3(0≤x≤1) Phases : a synthesis, structure, and electron microscopy study
- 1998
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 141, s. 514-521
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Tidskriftsartikel (refereegranskat)abstract
- Reduced niobates Ca1−xSrxNbO3(0≤x≤1) with perovskite-type structures have been synthesized at 1500°C in niobium ampoules sealed under argon gas. The prepared compounds were characterized by X-ray powder diffraction, electron diffraction, high-resolution electron microscopy, and energy-dispersive X-ray analysis. The structure of CaNbO3(GdFeO3type) was refined using X-ray powder diffraction data. Electron diffraction studies showed that complex superstructures of the perovskite-type structure occur forx>0 in Ca1−xSrxNbO3: (i) forx=0.2 and 0.3 reflections corresponding to a supercell witha≈2×√2×aper,b≈4×aper, andc≈2×√2×aper(per=ideal perovskite) were observed, (ii) forx=0.5, 0.6, and 0.7 a cubic supercell was found witha=4×aper, whereas (iii) forx=0.8 and 0.9 the supercell is probably orthorhombic witha≈c≈√2×aperandb≈2×aper. Forx=1.0 a new orthorhombic modification of SrNbO3witha≈c≈√2×aperandb≈2×aperwas found. Resistivity measurements showed CaNbO3to have a nonmetallic type of conductivity, whereas the Sr-containing samples were metallic.
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| 10. |
- Istomin, S Y, et al.
(författare)
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Synthesis and characterization of reduced niobates CaLnNb2O7, Ln = Y, Nd with a pyrochlore structure
- 1997
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Ingår i: Materials research bulletin. - 0025-5408 .- 1873-4227. ; 32:4, s. 421-430
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Tidskriftsartikel (refereegranskat)abstract
- The first known reduced niobates with a pyrochlore structure, CaLnNb(2)O(7) Ln = Y, Nd, have been synthesized from Ca2Nb2O7, Ln(2)O(3), and NbO2 in sealed Nb ampoules heated at 1400 degrees C for 6 h. Our studies show that these compounds have a small homogeneity region according to the formula Ca(2-x)Ln(x)Nb(2)O(7), Ln = Nd with x = 0.9, 1.0; and Ln = Y with x = 1.0, 1.1. The prepared compounds were characterized by EDX (energy dispersive X-ray) analysis, X-ray and electron diffraction, and thermogravimetric analysis. The crystal structures of CaNdNb2O7 and CaYNb2O7 were refined using X-ray and neutron powder diffraction data. CaNdNb2O7 and CaYNb2O7 have undistorted pyrochlore structures: space group Fd3m, a = 10.416(1) Angstrom and 10.3159(8) Angstrom, respectively. Resistivity measurements show that CaNdNb2O7 exhibits semiconducting behavior. Copyright (C) 1997 Elsevier Science Ltd.
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